Add a public_file_prefix option to cashier.conf

Allow the client to save the public key and public cert to files
that start with public_file_prefix and end with .pub and -cert.pub
respectively.

This is the naming scheme the ssh IdentityFile config option supported
for certs starting in version 5.4p1. Starting in version 7.2p1, an
additional option, CertificateFile, was added, but the IdentityFile-only
method with those names still works.

Used in conjunction with a user's ~/.ssh/config file setting
IdentitiesOnly and IdentityFile, this change will allow for multiple
ssh CAs for different services.

Note that this will resolve #49 .